When a user starts Word manually, they activate the instance of Word that
you previously created using CreateObject, instead of opening a new instance

If you start an instance of Word using CreateObject, and don't make it
visible, and if a user subsequently starts Word manually, they will activate the
instance of Word you created programmatically, instead of opening a new
instance. However, if you make Word visible immediately after creating the
object, you don't get the problem.

The workaround (assuming you don't want to make Word visible) is to create a
temporary Word object, then create the
“real”
one, then close the temporary object. This is also covered in Microsoft
Knowledge Base article
Q188546, which includes the following code sample: